In the above technical field, patent literature 1 describes detection of an abrupt frequency change by measuring a fluctuation in a phase in the time direction. Patent literature 2 describes, in paragraph 0031, that “a phase linearizer 25 corrects a hop in a phase signal θ input from a polar coordinate converter 24 by linearization and outputs a resultant phase signal θ′ to a phase detector 26”. Non-patent literature 9 describes detecting an abrupt signal change using the frequency-direction linearity of a phase.